1. Field of the Invention
The present invention relates to a piezoelectric printing head in which a piezoelectric actuator is used.
As the printing speed of impact dot-matrix printers has increased, electromagnetic type printing heads can not be applied to such high speed printers. Therefore, printing heads using piezoelectric actuators have been reconsidered. Under these situations, however, the amount of displacement caused by the expansion and contraction of a piezoelectric element is very small, that is, the amount of displacement is only several microns. Therefore, this small displacement is magnified by a magnifying mechanism so that a printing operation can be carried out. In this case, it is necessary to prevent damage to a beam in the magnifying mechanism and the overshooting of a wire so that the printing quality can be improved and the life of the printing head can be extended.
2. Description of the Related Art
A conventional piezoelectric printing head includes a common base and piezoelectric elements, the piezoelectric printing head, an armature, and a magnifying mechanism. A plurality of piezoelectric elements are circumferentially provided in predetermined positions, and the armature and magnifying mechanism are provided corresponding to each piezoelectric element. The piezoelectric element has a base side block which is secured to the base by means of adhesion.
The armature has a beam, and a printing wire is secured to a tip of the beam. The magnifying mechanism comprises a support spring and a movable spring that are disposed in parallel with each other. The upper end portions of the springs are secured to the armature, and the lower end portion of the support spring is secured to the base, and that of the movable spring is secured to the piezoelectric element base. The piezoelectric printing head is mounted on a carrier, and moved on a platen together with the carrier. Printing is conducted when a predetermined piezoelectric element is driven at a predetermined time in the process of moving.
When a voltage is applied to a piezoelectric element, the piezoelectric element is displaced upward. Then the magnifying mechanism is activated, and the armature is rotated, so that the wire is forced to conduct a printing motion. When the printing has been completed, the supply of voltage impressed upon the piezoelectric element is removed and the armature returns to the initial position together with the piezoelectric element.
When the piezoelectric printing head in the aforementioned conventional structure is driven at high speed, stress concentration is caused in a joint portion between a leaf spring of the magnifying mechanism and the armature, so that the leaf spring is damaged. In order to relieve the stress concentration of each leaf spring, it is necessary to reduce a ratio of magnification or a characteristic frequency of the magnifying mechanism. However, magnification loss occurs in the aforementioned case, so that it is not possible to cope with high speed printing.
Also, in the aforementioned conventional structure, when the movable spring is pushed up by the displacement of the piezoelectric element, an adhesive agent connecting the piezoelectric element with the base is deformed by the reaction force. Therefore, the ratio of magnification or the characteristic frequency of the magnifying mechanism is lowered, so that a magnifying loss is caused and high speed printing cannot be carried out.
Further, in the conventional printing head having the aforementioned structure and function, the width of the beam joint portion is the same as that of the armature, so that a stress concentration is caused in a position close to the upper and lower end portions of the beam joint portion. For that reason, the beam thickness cannot be reduced, and the printing speed cannot be increased.